Diagnostic elements are important components of clinically relevant analytical methods. In this connection, the primary focus is on the measurement of analytes, e.g. metabolites or substrates, which are for example determined directly or indirectly with the aid of an enzyme which is specific for the analyte. In this case, the analytes are converted with the aid of an enzyme-coenzyme complex and subsequently quantified. In this process, the analyte to be determined is brought into contact with a suitable enzyme, a coenzyme and optionally a mediator and the coenzyme is physicochemically changed by the enzymatic reaction, e.g. oxidized or reduced. If a mediator is additionally used, this mediator usually transfers the electrons released during the reaction of the analyte from the reduced coenzyme to an optical indicator or to the conductive components of an electrode so that the process can for example be detected photometrically or electrochemically. A calibration yields a direct relationship between the measured value and the concentration of the analyte to be determined.
Diagnostic elements known from the prior art are characterized by a limited shelf-life and by special requirements for the environment, e.g. cooling or dry storage, in order to achieve this shelf-life. Thus, in certain forms of application, e.g. in the case of tests which are carried out by the end user himself such as blood glucose self-monitoring, erroneous results may occur due to a false, unnoticed incorrect storage of the measurement system which can hardly be recognized by the consumer and may lead to an erroneous treatment of the respective disease. The erroneous results are primarily due to the fact that the enzymes, coenzymes and mediators used in such diagnostic elements generally react sensitively to moisture and heat and are inactivated over time.
A known measure which is used to increase the stability of diagnostic elements is the use of stable enzymes, for example the use of enzymes from thermophilic organisms. Furthermore, enzymes can be stabilized by chemical modification, and in particular by cross-linking. Moreover, enzyme stabilizers such as for example trehalose, polyvinyl pyrrolidone and serum albumin can also be added, or the enzymes can be enclosed in polymer networks by photopolymerization for example.
Another method of stabilizing enzymes is by means of mutations that are introduced site-specifically or non-site-specifically. In this connection, the use of recombinant techniques which specifically influence the properties of the corresponding enzyme by means of a targeted change in the DNA coding for an enzyme, have proven to be particularly suitable.
Baik et al. (Appl. Environ. Microbiol (2005), 71, 3285) describe the isolation and characterization of three mutants of glucose dehydrogenase from Bacillus megaterium which contain the amino acid substitutions E170K, Q252L or E170K/Q252L. Whereas the mutants E170K and Q252L only have a low stability at low salt concentrations and high pH values, the double mutant exhibits a significantly increased stability under the test conditions due to an enhanced interaction at the dimer-dimer interface.
Vázquez-Figueroa et al. (ChemBioChem (2007), 8, 2295) disclose the development of a thermostable glucose dehydrogenase which comprises introducing amino acid substitutions at positions 155, 170 and 252 of the glucose dehydrogenase from Bacillus subtilis, Bacillus thuringiensis and Bacillus licheniformis. In this connection, it is stated that the mutations E170K and Q252L, individually as well as in combination, result in a stabilization of glucose dehydrogenase from Bacillus subtilis. 
However, when stabilized enzymes which are genetically modified compared to the wild-type variant are used, the problem arises that they usually have a considerably lower activity than the corresponding wild-type enzyme, and thus cause a lower substrate turnover per unit of time. If one takes into consideration the fact that enzymes having a high specific activity are preferably used in clinical and diagnostic chemistry such as in the detection of blood glucose, the use of stabilized enzymes is often an unacceptable alternative to the use of native enzymes.
Another difficulty is that high enzyme activities which correlate with a high substrate turnover per unit of time are usually only achieved with the respective native coenzyme in each case. If an artificial coenzyme is used instead of the native coenzyme, then the enzyme activity is usually drastically reduced and the rate of turnover with the substrate decreases accordingly.